why when I cook a a rib roast at 250 degress for 4 hours to a temp of 130 degrees when I slice the roast the color is red but in 1-2 minutes it turns

My Mom who was a culinary expert, always patted the roast with olive oil, salt and pepper before putting in the oven at high temperature, then lowering it. I think it has to do with creating a crust to lock in juices.

What doneness are you trying to achieve? If medium-rare to medium, remove your roast from the oven when a thermometer at the center reads 118-120. The center cuts will be medium-rare, and the ends medium. Always give meat a rest period before slicing. The larger the cut, the longer the rest. For a rib roast, cover it with foil and let it rest for 20-30 minutes. Resting allows water molecules which have been squeezed out of cells by the stress of cooking, to re-enter the cells, plumping them up, and allowing a greater tenderness. If any meat is sliced immediately upon leaving the oven, you will see liquid flow right out, and the meat will have a drier texture.